1. Background of the Invention
The present invention is related to the field of fiber optic communications and networks, and more particularly, provides devices, filter systems, and methods for filtering of optical signals, especially for use in dense wavelength division multiplex systems.
A variety of optical filters have been developed to differentiate optical signals based on their wavelength. For example, thin film dichroic optical filters can selectively pass signals having wavelengths that are longer or shorter than a nominal wavelength. Specifically, low pass thin film filters selectively pass optical signals having a wavelength shorter than a maximum wavelength. High pass thin film filter, sometimes called long pass filters, pass signals with wavelengths that are longer than a minimum wavelength. Multi-cavity thin film filters are now in use to selectively pass light signals within a limited range. Such bandpass filters having fairly narrow wavelength transmission ranges (also called pass bandwidths) can currently be produced, but usually at quite high costs.
Unfortunately, thin film bandpass filters do not always provide the desired filtering performance for dense wavelength division multiplex systems. Specifically, some portion of the optical signal which is outside of the nominal pass range of a multi-cavity narrow bandpass filter typically "leaks through" with the filtered signal. This leakage can lead to cross-talk between signals of different wavelengths within the multiplexed system. To avoid cross-talk between signals of differing wavelengths, known fiber optic data transmission systems generally separate the nominal wavelengths of the multiplexed signals by about 0.8 nm or more.
Although these known wavelength division multiplex (WDM) systems are quite effective, allowing large amounts of data to be transmitted over a single optical fiber, data transmission capabilities of optical fibers would benefit significantly if the number of signals transmitted along a fiber could be increased. Fiber data transmission capabilities can be enhanced by increasing the "density" of the multiplexed signals, that is, by decreasing the separation between the discrete frequencies or wavelength of the multiplexed signals. WDM systems of increased density will require better filters.
In general, to minimize cross-talk between adjacent signals, high performance optical bandpass filters should exhibit a high transmission throughout the desired pass bandwidth, a very low transmission of signals having wavelengths outside the desired pass bandwidth, and a very narrow transition bandwidth between the high transmission and low transmission ranges. A high performance filter having a narrow spectral transition zone is said to have a "steep skirt," as can be understood by viewing a graph of the transmission response over a range of wavelengths or frequencies. As a portion of the signal will be transmitted and a portion of the signal will be blocked within this skirt region, it is particularly desirable to minimize the bandwidth of this transition region in high performance filters for dense wavelength division multiplex systems so as to minimize cross-talk. High performance bandpass filters should also provide an even or "flat-top" transmission spectrum throughout the pass bandwidth, with very small (or no) ripples atop the transmission spectral shape.
Several different filter structures have been proposed in previous attempts to increase the channel density of multiplex systems. Work in connection with the present invention has shown that it is possible to fabricate high performance thin film filters having a large number of coating layers so as to provide multiple resonator cavities. While such multi-resonator thin film filters can provide fairly good performance, manufacturing limitations make it extremely difficult to fabricate these thin film structures with the desired combination of a narrow pass bandwidth, a sharp skirt, and an accurate center wavelength. Additionally, thin film filters having four or five resonator cavities may include as many as 150 thin film layers. The deposition of each of these layers stresses the underlying structure, and the release of this stress after fabrication is complete may shift the wavelength response of the filter. This Limits the temperature stability and lifetime of these expensive structures, while the large number of layers may result in relatively high signal loss and an undesirable ripple in the passband of the transmission spectrum. As a result, multi-resonator multi-layer thin film coated filter structures are plagued by a low manufacturing yield, a high failure rate, and a very high cost, while still often falling short in some of the important requirements for filtering of dense wavelength division multiplex systems.
A much lower cost structure for selectively filtering optical signals is the Fiber Bragg Grating. However, each Fiber Bragg Grating is a reflective device that is generally limited to a very narrow reflective bandwidth. When a large number of these gratings are combined to perform as a bandpass filter, the gratings often extend along an excessive fiber length. Other proposed Fiber Bragg Grating-based filtering structures require precise (and expensive) phase alignment and couplers. Moreover, these proposed systems re-inject the pass band wavelength into the signal input fiber (so that additional, and often expensive, optical fiber devices are included to effect bandpass transmission from these reflective gratings), and are generally sensitive to temperature variations.
In light of the above, it would be desirable to provide improved optical filter devices, systems, and methods for separating optical signals of differing wavelengths. It would be particularly desirable to provide improved bandpass and dichroic filters for use in dense wavelength division multiplexed systems and networks. It would further be desirable if these improved structures and methods provided steep skirts for wavelength signal multiplexing and de-multiplexing, wavelength routing, switching, connecting, and other multiple wavelength operations. These improved filtering techniques should ideally exhibit accurate operation wavelengths, low insertion loss, low ripples, near zero temperature effects, low polarization dependent loss, high return loss, flat pass band response, and a skirt sufficiently sharp to minimize channel cross-talk when the wavelength separation between optical signals is decreased below that of existing dense wavelength division multiplexed systems.